You owe a certain person a sum of money.

你欠着某人一笔钱。

A certain person called on me yesterday.

昨天有人来找过我。

A certain person called on me yesterday.

某个人昨天拜会了我。

A certain person called on you yesterday.

昨天有个人来拜访过你。

Everything's going well until a certain person comes in.

一切进展顺利，直到某个人的到来。